System and method for cooling a body

ABSTRACT

A system and method for cooling an external surface of a heat conductive body heated by a heat source is provided. The system includes a tank containing an evaporation liquid, a conduit supplying the evaporation liquid to the external surface of the body, a controllable supply valve for regulating a flow rate of egress of the evaporation liquid from the tank, and a control unit for controlling operation of the supply valve. The control unit includes a temperature sensor producing a temperature sensor signal representative of the temperature of the body at the predetermined place; and a controller capable of generating control signals for controlling operation of the controllable supply valve to provide a pulsed supply of the evaporation liquid to the external surface of the body for obtaining a desired temperature decrease of the body.

TECHNOLOGICAL FIELD

This invention relates to cooling systems, and in particular, to asystem and method for cooling an external surface of a body, which isheated by a heat source and located in a low-pressure environment.

BACKGROUND

Systems are known in the art, which are designed for cooling objectslocated in a low-pressure environment. For example, during spaceexploration, it is important to control the temperature of the surfaceand the body of the space vehicle in order to dissipate the waste heatgenerated by electronics of the space vehicle, and any heat absorbed bythe vehicle shell due to external radiation emanated from the sun.Likewise, it is important to control the temperature of a spacesuit todissipate the metabolic heat of the suit inhabitant.

For example, U.S. Pat. No. 3,212,286 describes a spacesuit having athermally conductive wall with a plurality of porous metal blockevaporators distributed over and in good thermal contact with theexterior of the wall. The metal blocks act as constant temperature heatsinks, dissipating into space the heat transferred to them by the suitwall.

The evaporators are supplied by a water evaporant from a reservoirwithin the spacesuit, the water being under the atmospheric pressure ofthe suit. On first entering the evaporator, the water does notimmediately freeze, due to its own vapor pressure, and wetting of theinterior surfaces of the evaporator block occurs. Then, the waterfreezes and then sublimes to remove heat from the block. As sublimationproceeds, passages open to admit more water, thus sustaining the coolingprocess.

U.S. Pat. No. 3,613,775 describes a cooling system for removingmetabolic heat in a life support system used in space activity. Inoperation, coolant passes through a sublimator which also includes apump and a coolant garment. A separate water storage container providesfeedwater to the sublimator where it is sublimated along a surfacethermally connected to a heat exchange element through which the coolantflows.

U.S. Pat. No. 5,092,129 describes an apparatus for removing heat energyof a cooling medium passing from a spacesuit liquid cooling garment. Theapparatus includes a heat sink assembly for absorbing and rejecting theheat energy and a heat transfer means for transferring the heat energyof the cooling medium to the heat sink assembly. The heat transfermeans, which is comprised of an array of thermoelectric modules,regulates the quantity of heat energy transferred from the coolingmedium to the heat sink assembly. The heat sink assembly includes amaterial that isothermally changes phase while absorbing heat energy.

GENERAL DESCRIPTION

Despite prior art in the area of cooling for cooling a body, which islocated in a low-pressure environment, there is still a need in the artfor further improvement in order to provide a novel system that canprovide controllable temperature decrease of the body at a predeterminedplace.

The present invention satisfies the aforementioned needs in the art byproviding a novel system for cooling an external surface of a heatconductive body heated by a heat source.

According to an embodiment of the present invention, the cooling systemincludes a tank containing an evaporation liquid having a pressuregreater than a pressure of an environment near the external surface. Theenvironment can, for example, be a low-pressure environment, such asvacuum or upper layers of the earth's atmosphere.

The cooling system also includes a conduit being in hydrauliccommunication with the tank and configured to supply the evaporationliquid to the external surface of the body.

The cooling system also includes a controllable supply valve arrangedwithin the conduit. The controllable supply valve is configured forregulating a flow rate of egress of the evaporation liquid from thetank.

The system further includes a control unit operatively coupled to thecontrollable discharge valve. The control unit is configured forcontrolling operation thereof for supply of the evaporation liquid. Thecontrol unit includes a temperature sensor arranged within the body at apredetermined place and configured for producing a temperature sensorsignal representative of the temperature of the body at thepredetermined place, and a controller operatively coupled to thetemperature sensor and to the controllable supply valve. The controlleris responsive to the temperature sensor signal and is capable ofgenerating control signals for controlling operation of the controllablesupply valve. Controlling operation of the controllable supply valve iscarried out by turning it “on” or “off”, thereby providing a pulsedsupply of the evaporation liquid to the external surface of the body aslong as required for obtaining a desired temperature decrease of thebody at the predetermined place.

According to an embodiment of the present invention, the control unitincludes a flow meter and a flow rate valve arranged in the conduit. Theflow meter is configured for measuring the flow rate of the evaporationliquid in the conduit, and the flow rate valve is configured to regulatethe flow rate.

According to an embodiment of the present invention, the pulsed supplyof the evaporation liquid is characterized by a duration Δt of theliquid supply pulses and an operating frequency of the controllablesupply valve.

According to an embodiment of the present invention, the duration Δt ofeach pulse of the pulsed supply of the evaporation liquid is obtained byΔt=McΔT/LJ, where M is the mass of the body, c is the specific heatcapacity of the body, ΔT is the desired temperature decrease; L is thelatent heat of evaporation of the evaporation liquid, and J is the flowrate of the evaporation liquid through the conduit.

According to an embodiment of the present invention, an operatingfrequency of the controllable supply valve is obtained by f=W/LJΔt,where W is the heat power of the heat source.

According to an embodiment of the present invention, an inner volume ofthe conduit between the controllable supply valve and an opening throughwhich the evaporation liquid is supplied to the external surface of thebody has a predetermined value obtained by ΔV=McΔT/Lρ, where ρ is thedensity of the evaporation liquid.

According to an embodiment of the present invention, the cooling systemfurther includes a grid arranged over the external surface of the heatconductive body at a predetermined distance. The grid is configured forcatching the evaporation liquid provided by the conduit, and holding theevaporation liquid in a gap between the grid and the external surface.

The present invention also satisfies the aforementioned needs in the artby providing a novel method for cooling an external surface of a heatconductive body. The method includes controlling operation of thecontrollable supply valve by turning it “on” or “off”, thereby providinga pulsed supply of the evaporation liquid to the external surface of thebody as long as required, for obtaining a desired temperature decreaseof the body at the predetermined place.

The system for cooling of the present invention has many of theadvantages of the prior art techniques, while simultaneously overcomingsome of the disadvantages normally associated therewith.

The system for cooling of the present invention can be used for a broadrange of cooling purposes. These include cooling relatively small heatsources, such as small electronic components. For such a purpose, thecooling system can be used with a heat conductive body abated to theelectronic component, such as a relatively thin aluminum plate having aconduit terminated with one or more orifices on its external surface.The cooling unit can be few millimeters thick. This means that it can beattached to the heat source with almost no total volume increase. Thismodularity also allows simple replacement of any damaged components ofthe cooling system.

Since a conduit can be very narrow (i.e., with a diameter less than amillimeter), for certain heat source designs, the conduit itself can beengraved in the heat source bulk or skin.

For relatively large heat sources, a plurality of heat conductive bodiescan be attached to the heat source around the outer surfaces foreffective cooling.

The cooling system according to the present invention may be readilyconformed to complexly shaped surfaces and contours of a body. Inparticular, it can be readily conformable to various three dimensionalbodies having complex non-uniform shapes.

The cooling system according to the present invention may be efficientlymanufactured.

The installation of the cooling system to a body is relatively quick andeasy and can be accomplished without substantial altering the platform,in which it is to be associated.

The cooling system according to the present invention is of durable andreliable construction.

There has thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows hereinafter may be better understood. Additional detailsand advantages of the invention will be set forth in the detaileddescription, and in part will be appreciated from the description, ormay be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosedherein and to exemplify how it may be carried out in practice,embodiments will now be described, by way of non-limiting example only,with reference to the accompanying drawings, in which:

FIG. 1A illustrates a general schematic view of a cooling system,according to one embodiment of the present invention;

FIG. 1B illustrates a general schematic view of a cooling system,according to another embodiment of the present invention;

FIG. 2 illustrates a general schematic view of a cooling system,according to a further embodiment of the present invention; and

FIG. 3 illustrates exemplary graphs depicting the time dependence of theheating power supplied to the body, and the temperature of the bodymeasured at four different places within the body during operation ofthe cooling system shown in FIG. 2.

DETAILED DESCRIPTION OF EMBODIMENTS

The principles and operation of the cooling system and method forcooling an external surface of a heat conductive body according to thepresent invention may be better understood with reference to thedrawings and the accompanying description, it being understood thatthese drawings and examples in the description are given forillustrative purposes only and are not meant to be limiting. The samereference Roman numerals and alphabetic characters will be utilized foridentifying those components which are common in the cooling system andits components shown in the drawings throughout the present descriptionof the invention. It should be noted that the blocks in the drawingsillustrating various embodiments of the present invention are intendedas functional entities only, such that the functional relationshipsbetween the entities are shown, rather than any physical connectionsand/or physical relationships.

Some portions of the detailed descriptions, which follow hereinbelow,are presented in terms of algorithms and/or symbolic representations ofoperations on data represented as physical quantities within registersand memories of a computer system. An algorithm is here conceived to bea sequence of steps requiring physical manipulations of physicalquantities and leading to a desired result. Usually, although notnecessarily, these quantities take the form of electrical or magneticsignals capable of being stored, transferred, combined, compared, andotherwise manipulated. In the present description, these signals can bereferred to as values, elements, symbols, terms, numbers, or the like.

Unless specifically stated otherwise, throughout the description,utilizing terms such as computing or “calculating” or determining or“obtaining” or the like, refer to the action and processes of a computersystem, or similar electronic processing device, that manipulates andtransforms data.

According to the present invention, the cooling of a heat conductivebody is based on applying discrete doses (pulses) of evaporation liquid(evaporant) onto an external surface of the body. In the presentapplication, the terms “evaporant” and “evaporation liquid” are usedinterchangeably.

According to an embodiment, the external surface of the body is exposedto a low-pressure environment. The term “low-pressure” is broadly usedherein to refer to any pressure less than the saturated vapor pressureof the evaporation liquid used in the system of the present invention,i.e., to any pressure at which a vapor of the evaporation liquid is inunsaturated state.

Examples of a low-pressure environment include, but are not limited to,a space vacuum and upper layers of the earth's atmosphere. The term“body” is broadly used in the present description and the claims todescribe any object having at least one surface exposed to alow-pressure environment. Examples of a body with an external surface(shell) exposed to a low-pressure environment include, but are notlimited to, a spacesuit and a space vehicle located in the space vacuumor in the upper layers of the earth's atmosphere. Examples of a bodywith an external surface (shell) exposed to a low-pressure environmentalso include, various modules (electrical, optical, mechanical) of anysystem in the space vacuum or in the upper layers of the earth'satmosphere.

Once the evaporant reaches the external surface it evaporates, andthereby cools the body by drawing the heat away from the surface of thebody. For example, an evaporant such as water has a latent heat ofvaporization of 2461 kJ/kg, which makes evaporative cooling with wateran effective process for dissipating unwanted heat.

Ability to control the dose size and frequency of the evaporant dosepulses enables controlling the cooling power applied to the body. Forinstance, relatively small and frequently applied doses of anevaporation liquid can be applied to achieve very accurate temperaturecontrol for small heat power systems. On the other hand, large doses canbe used when the body is heated by a great heating power. The coolingcontrol scheme of the present invention resembles a pulse widthmodulation (PWM) control scheme, providing controlling of duration ofthe evaporant supply (pulse width) and frequency of the evaporantsupply.

Referring now to FIG. 1A, a schematic view of a cooling system 10 forcooling an external surface 11 of a heat conductive body 12 heated by aheat source (not shown) is illustrated, according to one embodiment ofthe present invention. Arrows in FIG. 1A illustrate heat transfer to theheat conductive body 12. The body 12 can be heated by any heat transfermechanism. For example, a heat source can be directly attached to thebody 12, and provide heat transfer by a heat conduction mechanism.Alternatively, a heat source can radiate heat energy towards the body12, and heat the body 12 by a heat radiation mechanism. Likewise, whenthe body 12 is separated from a heat source by a fluid (e.g., liquid orgas), heat transfer in can be implemented via a convection mechanism. Itshould also be understood that the heat source can be integrated withthe body 12 and be a part of it.

According to an embodiment, the cooling system 10 includes a tank 13comprising an evaporation liquid 14. The evaporation liquid in the tank13 has a pressure greater than the pressure of the environment near theexternal surface 11. It is preferable, but not mandatory, that the wholebody 12, or at least its external surface 11, is located in alow-pressure environment.

The term “tank” is broadly used to describe any container, chamber orvessel that can be used for holding the evaporation liquid 14 at adesired pressure. The tank 13 has an inlet port 131 for filling the tankand an outlet port 132 for releasing the evaporation liquid 14.

The material of the evaporation liquid affects the wettingcharacteristics of the external surface 11. Therefore, depending on thematerial of the external surface 11, the evaporation liquid is selectedhaving molecules attractable to the molecules of the external surface11. Moreover, density and viscosity of the evaporation liquid effect itsflow rate J. Other physical properties, for example, latent heat ofevaporation of the evaporation liquid, and chemical properties, such astoxicity or corrosion, should also be taken into account in selection ofthe material for the evaporation liquid 14. Examples of an evaporationliquid suitable for the purpose of the present invention include, butare not limited to, water, methanol, ethanol, acetone, etc.

The cooling system 10 also includes a conduit 15 being in hydrauliccommunication with the outlet port 132 of the tank 13, and configured tosupply the evaporation liquid 14 to the external surface 11 of the body12. As illustrated in FIG. 1A, the conduit 15 is formed as a channelwithin the body 12 and includes one or more outlet orifices 16 to theexternal surface 11, however other configurations of the conduit 15 arealso contemplated.

The cooling system 10 also includes a controllable supply valve 17arranged within the conduit 15, and configured for regulating a flowrate of egress of the evaporation liquid 14 from the tank 13. The term“valve” as used herein has a broad meaning and relates to any electricalor mechanical device adapted to controllably regulate the flow rate ofthe fluid.

The cooling system 10 is controlled by a control unit 18. The controlunit 18 is operatively coupled to the controllable discharge valve 17,and is configured for controlling operation thereof for supply of theevaporation liquid 14. The control unit 18 includes a temperature sensor181 arranged within the body 12 at a predetermined place and isconfigured for producing a temperature sensor signal representative ofthe temperature of the body at the predetermined place. The controllabledischarge valve 17 and the temperature sensor 181 may be commerciallyavailable components.

The control unit 18 also includes a controller 182 operatively coupledto the temperature sensor 181 and the supply valve 17. Specifically, thesignals produced by the temperature sensor 181 can be relayed to thecontroller 182 via a connecting cable 183 or wirelessly. The controller182 is responsive to the temperature sensor signal and is capable ofgenerating control signals for controlling operation of the controllablesupply valve 17 by turning it “on” or “off”, thereby providing a pulsedsupply of the evaporation liquid to the external surface 11 of the body12. For example, if the temperature of the body is higher than therequired temperature, the controller 182 can produce a temperaturecontrol signal to activate operation of the controllable supply valve toprovide a pulsed supply of the evaporation liquid to the externalsurface of the body. The pulsed supply of the evaporation liquid cancontinue as long as required for obtaining a desired temperaturedecrease of the body 12 at a predetermined place.

FIG. 1B is a schematic view of a cooling system 100 for cooling anexternal surface 11 of a body 12 heated by a heat source (not shown),according to another embodiment of the present invention. The coolingsystem 100 differs from the cooling system 10 in FIG. 1A by the factthat it includes conduit 15 that is formed as an external pipe 151. Theconduit pipe 151 is in hydraulic communication with the outlet port 132of the tank 13, and includes one or more nozzles 152 arranged at the endof the conduit pipe 151 which are used to provide a jet of theevaporation liquid 14 onto the external surface 11 of the body 12.Preferably that in this case the nozzles 152 are close to or directly intouch with the external surface.

A method for cooling the external surface 11 of the heat conductive body12 includes controlling operation of the controllable supply valve 17 byturning it “on” or “off”. The operation starts when the controllablesupply valve 17 is turned off (closed) and there is no evaporationliquid 14 in the conduit 15. The body 12 is heated by the heat which istransferred from the heat source providing heat power W. The temperaturesensor 181 measures temperature T of the body at the predetermined placeand produces a temperature sensor signal representative of thetemperature. This temperature sensor signal is relayed to the controller182. When the temperature of the body changes to a predetermined valueΔT, the controller 182 generates a control signal for controllingoperation of the controllable supply valve 17 by turning it on (opening)in order to cool the body down.

According to an embodiment of the invention, the controllable supplyvalve 17 is opened for a predetermined time interval Δt. During the timeinterval Δt the evaporation liquid contained in the conduit 15 isreleased from the tank 13 and supplied to the external surface 11 of thebody 12 due to suction of the evaporation liquid. Such suction occursdue to a pressure gradient between the tank 13 and the external surface11, which is exposed to a low-pressure environment. As the evaporationliquid arrives at the external surface, it evaporates, and thereby drawscorresponding heat from the body. As a result, the temperature of thebody decreases by ΔT. Thus, the value of ΔT can serve as a temperaturestabilization resolution of the temperature stabilization method.

According to an embodiment of the invention, the cooling system 10 canincludes a flow meter (not shown) and a flow rate valve (not shown)arranged in the conduit 15. The flow meter is configured for measuringthe flow rate J of the evaporation liquid through the conduit 15. Theflow rate valve is configured to regulate the flow rate J. The flow ratecan, for example, be regulated “on the fly”, i.e., during operation ofthe cooling system.

According to another embodiment of the invention, the flow rate can bepre-calibrated by using flow meter and the flow rate valve during thefabrication and/or installation of the cooling system. In this case, theflow meter and the flow rate valve are not used after installation ofthe system and therefore may not be included in the system.

The amount of heat ΔQ drawn from the body away during the time intervalΔt equals to ΔQ=JLΔt. This heat ΔQ is used to decrease the temperature Tof the body 12 by ΔT and can be obtained by ΔQ=McΔT, where M is the massof the body 12, c is the specific heat capacity of the body 12, and L isthe latent heat of evaporation of the evaporation liquid 14.

According to an embodiment of the invention, the time interval Δt (whichis obtained by Δt=McΔT/LJ) can be used as a duration of the openposition of the valve in the pulse regime of supply of the evaporationliquid.

After operation that continued during the time interval Δt, thecontrollable supply valve 17 is closed (turned off), therebyinterrupting supply of the evaporation liquid. During the time intervalΔt when the supply valve 17 is opened, the temperature T of the body hasbeen decreased back by ΔT. When the controllable supply valve 17 is inthe closed position, the temperature of the body 12 starts again toincrease due to heat transferred from the heat source. In order todecrease the temperature, the controllable supply valve 17 should beopened again.

In order to stabilize the temperature of the body, the total coolingpower should be equal to the heating power. According to an embodimentof the invention, in order to maintain the temperature of the bodyaround a certain value, the supply pulses of the evaporation liquidshould be provided as a series of pulses, and the supply valve 17 shouldoperate with a certain duty cycle. In other words, the liquid supplypulses are separated from each other by a pause, when the supply valve17 is closed.

According to an embodiment of the invention, the operating frequency ofthe controllable supply valve (i.e., the repetition rate of the liquidsupply pulses) can be obtained by f=W/LJΔt, where W is the heat power ofthe heat source.

It should be noted that once the evaporation liquid passes through theconduit 15, it cannot be “pulled back”. Therefore, the evaporationliquid that is located in the inner volume ΔV of the conduit 15 betweenthe controllable supply valve 17 and the outlet orifice(s) 16 of theconduit 15 cannot be controlled by the controllable supply valve 17, andis inevitably sucked out to the external surface 11 and evaporates.Thus, the resolution of the cooling power of the system is, inter alia,determined by this “dead dose” of the evaporation liquid located withinthe conduit 15 between the controllable supply valve 17 and the outletorifice(s) 16.

According to an embodiment of the invention, the design of the coolingsystem takes into account the “interplay” between the inner volume ΔV ofthe conduit 15 between the controllable supply valve 17 and the outletorifice(s) 16 and the volume Δv of the evaporation liquid released bythe controllable supply valve 17 during the time interval Δt, i.e., thevolume of the evaporation liquid passing through the controllable supplyvalve 17 in the pulse regime.

In particular, when Δv<<ΔV, each valve opening releases a relativelysmall amount of evaporation liquid into a relatively large volume of theconduit. In this case, the evaporation liquid may not reach the outletorifice(s) 16 and evaporate within the conduit 15. Such evaporation maycause significant decrease of temperature within the conduit 15 that canresult in freezing of the evaporation liquid, and thereby blocking thepassage in the conduit 15 with ice. Moreover, the flow dynamic of thesmall volume of evaporation liquid inside the large volume of theconduit is unpredictable, especially if the conduit has more than oneorifice.

On the other hand, when Δv>>ΔV, each valve opening releases a relativelylarge amount of evaporation liquid into a relatively small volume of theconduit. In this case, most of the released liquid is supplied to theexternal surface 11 of a body 12 and cools the body. It should beunderstood that this case is not practical, especially in the case whenthe conduit is tiny and therefore may be costly in fabrication.Therefore, there is no need in fabrication of such conduits.

Thus, according to an embodiment of the invention, the controllableoperation of the cooling system is optimally carried out when the volumeΔv of the evaporation liquid released by the controllable supply valve17 during the time interval Δt is about the inner volume ΔV of theconduit 15 between the controllable supply valve 17 and the outletorifice(s) 16, i.e., Δv≈ΔV. In this case, the inner volume ΔV of theconduit between the controllable supply valve and an opening throughwhich the evaporation liquid is supplied to the external surface of thebody, has a predetermined value obtained by ΔV=McΔT/Lρ, where ρ is thedensity of the evaporation liquid.

The calculated value ΔV can be taken into account for design andmanufacturing of the conduit 15 and especially the portion of theconduit 15 between the controllable supply valve 17 and the outletorifice(s) 16.

In the present invention, the term “about” for the deviation of thevalue Δv from the value ΔV refers to a value, amount, or degree that isapproximate or near to each other. The term “about” means within astatistically meaningful range of a value and indicates a reasonableamount of deviation caused by the differences between, inter alia, theflow rate J of the evaporation liquid in different parts of the conduit15, that does not bring about a considerable change as a result. Theextent of variation of value Δv from the value ΔV encompassed by theterm “about” is that which is typical for the tolerance levels ormeasurement conditions. The allowable variation encompassed by the term“about” depends on the particular system under consideration, and can bereadily appreciated by one of ordinary skill in the art. Thisapproximation for the purpose of the present invention can, for example,be interpreted so as to include an error of 20% at least, as long asthere is no considerable change in the performance of the system due tothe deviation.

It should be understood that evaporation rate of an evaporation liquiddepends on several factors affecting the evaporation process.

In particular, if the environment surrounding the external surface ofthe body 12 already has a high concentration of the substance of theevaporation liquid or other substances, then the evaporation liquid 14released from the conduit 15 will evaporate at a slower rate.Furthermore, environment pressure of the gas surrounding the externalsurface of the body 12 affects the evaporation rate. Moreover, thetemperature and enthalpy of vaporization of the evaporation liquidaffects the evaporation rate.

It should also be understood that an evaporation liquid which is spreadover a larger surface area will evaporate faster, as there are moresurface molecules per unit of volume that are potentially able toescape. The area of the external surface 11 of the body 12 covered withthe evaporation liquid can, for example, be increased by enhancingwetting characteristics of the evaporation liquid. The wettingcharacteristics can, for example, be improved by treatment of theexternal surface 11 of the body.

According to an embodiment of the present invention, the surface iscovered by a layer (not shown in FIGS. 1A and 1B) of material havingmolecules attractable to the molecules of the evaporation liquid. Forexample, when the evaporation liquid is water, the external surface 11of the body 12 can be covered by a layer of a hydrophilic material.Examples of hydrophilic materials suitable for the purpose of thecooling system of the present invention include, but are not limited to,cellulose, polyamides, polyacrylic amides, polyurethanes withpolyethylene glycol ether soft segments, ethoxylated graft polymers,etc.

In certain conditions, such as in a vacuum and/or with zero gravity, itcan be complicated to apply evaporant to the external surface of thebody. For instance, in spray cooling by the system shown in FIG. 1B, theevaporant droplets are exposed to the vacuum as they leave the nozzle.Once a droplet is exposed to a vacuum, it starts to evaporate. Thetemperature of the droplet decreases, and it may freeze. Once thedroplet has frozen it may bounce back from the external surface uponarriving at the surface. In such a case, the cooling mechanism fails,since evaporation from the surface does not occur.

Likewise, in the case of zero gravity conditions, the evaporation liquidprovided by the conduit may not stick to the external surface as it isreleased from the outlet orifices 16 of the conduit in the system shownin FIG. 1A or from the nozzle 152 of the conduit in the system shown inFIG. 1B.

Referring to FIG. 2, a schematic view of a cooling system 20 for coolingan external surface 11 of a heat conductive body 12 heated by a heatsource (not shown) is illustrated, according to a further embodiment ofthe present invention. According to this embodiment, the cooling system10 includes a grid 21 arranged over the external surface 11 of the body12 at a predetermined distance. The grid 21 is configured for catchingthe evaporation liquid 14 provided by the conduit 15, and holding theevaporation liquid in openings 22 of grid 21, and in a gap 23 betweenthe grid 21 and the external surface 11. The grid has appropriatedimension of the openings 22, and is made from a suitable metal orplastic material, which can hold the evaporation liquid 14 in theopenings 22. Preferably, that the material of the grid is a wettablematerial. The openings 22 of the grid 21 can, for example, havedimensions of about 1×1 mm², while the gap 23 can be in the range ofabout 0.2 mm to 1 mm. In this case, the grid 21 can hold a sufficientlayer of evaporation liquid 14 caught between the grid 21 and theexternal surface 11, even at zero gravity, and provide the coolingprocess.

It should be noted that the diameter of the conduit should be largeenough so that the evaporation liquid may not freeze in the conduitlocated within the body before reaching the external surface. For theprovision shown in FIG. 1A, the mass and thermal conductivity of theheat conductive body should also be selected to avoid freezing of theevaporation liquid, since such freezing can prevent operation of thecooling control method described above.

In order to illustrate operation of the cooling system and method of thepresent invention, a prototype system has been fabricated. The prototypesystem has an aluminum heat conductive body having a rectangular prismshape of 50×43×6 mm³. A heat source that generates heat includes twoelectrical resistors connected to an aluminum plate having a rectangularprism shape of 50×43×3.5 mm³. The surface of the heat source plate,having dimensions of 50×43 mm², was attached to the surface of the heatconductive body having similar dimensions.

A low-pressure environment having pressure of 2 Torr was provided in avacuum chamber having dimensions of 600×600×600 mm³. Four thermocouples(temperature sensor) were mounted within the plate body in differentplaces.

A brass pipe was used as a conduit arranged within the body. Acontrollable supply valve was arranged in the conduit. A volume Δv ofthe conduit between the controllable supply valve and orifices of theconduit on the surface of the body was 150 mm³.

Water at the atmospheric pressure stored in a tank was used asevaporant. The constant flow rate J of 3 gram/sec was provided throughthe conduit. In operation, the controllable supply valve supplied waterpulses having duration (width) of Δt=50 milliseconds. The frequency ofthe pulses was varied to illustrate the dependency of the coolingtemperature of the body on frequency.

A grid formed from plastic wires of 0.3 mm diameter and having cell sizeof 1×1 mm² was mounted over the external surface of the plate bodyplaced in the vacuum chamber (together with the heat source). The gapbetween the grid and the external surface of the body was 0.5 mm. Awater layer formed in the gap can facilitate catching and holding thewater on the external surface until complete evaporation.

Experimental results obtained from the prototype system are illustratedin FIG. 3. Specifically, FIG. 3 illustrates exemplary graphs depictingthe time dependence of the heating power (curve 31) supplied to thebody, and the temperature (curve 32 a, 32 b, 32 c and 32 d) of the bodymeasured at four different places.

Each spike 33 indicates a single opening of the valve during Δt=50milliseconds.

In operation, during the first 140 seconds, the heat source operateswith variable heating power. After about 200 seconds, the temperaturereaches its maximum value of about 53 degrees, and then graduallydecreases by cooling the environment. Due to the applying of fourdiscrete evaporant doses, heat quantities drawn from the body are alsodiscrete, resulting in temperature steps, which are indicated by stagesa-d. These four stages of temperature decrease (each temperature drop ofapproximately 5 degrees) correspond to four openings of the valve, andare the manifestation of the parameter ΔT. Similar results oftemperature decrease are shown in stages e-g obtained during theinterval of operation of 1700-1800 sec. These temperature steps definethe temperature stabilization resolution of the prototype system. Inorder to provide a smaller temperature resolution, smaller temperaturedrops must be made. As explained above, this can, for example, beachieved by using a different evaporant, shorter valve opening time Δtand/or smaller volume Δv of the conduit.

At the moment corresponding to 430 seconds of operation, a 10 wattheating power was applied, and a sequence of openings of the valve wasperformed until the moment of 735 seconds. As can be seen, each timewhen the valve was open, the temperature dropped to about 25 degrees,while the temperature rose above 33 degrees when the valve was closed,with an attempt to stabilize the system occurring at about 30 degrees.The frequency of the water pulse in this time interval was about 1/55Hz. A similar result is seen during operation of the system between 870to 1070 seconds, where the stabilization temperature was about 12degrees.

During operation of the system between 1300 to 1500 seconds, a 30 wattheating power was applied. In this case, to stabilize the systemtemperature at about 30 degrees, the frequency of water pulses was 1/16Hz. Since in this case the heating power of the heating source was threetimes larger than heating power provided between 870 to 1070 seconds,the pulse frequency is also approximately three times higher. In thiscase, a temperature difference of about 3 degrees was observed betweenthe data provided by the thermocouples.

As such, those skilled in the art to which the present inventionpertains, can appreciate that while the present invention has beendescribed in terms of preferred embodiments, the concept upon which thisdisclosure is based may readily be utilized as a basis for the designingof other structures, systems and processes for carrying out the severalpurposes of the present invention.

Also, it is to be understood that the phraseology and terminologyemployed herein are for the purpose of description and should not beregarded as limiting.

Finally, it should be noted that the word “comprising” as usedthroughout the appended claims is to be interpreted to mean “includingbut not limited to”.

It is important, therefore, that the scope of the invention is notconstrued as being limited by the illustrative embodiments set forthherein. Other variations are possible within the scope of the presentinvention as defined in the appended claims. Other combinations andsub-combinations of features, functions, elements and/or properties maybe claimed through amendment of the present claims or presentation ofnew claims in this or a related application. Such amended or new claims,whether they are directed to different combinations or directed to thesame combinations, whether different, broader, narrower or equal inscope to the original claims, are also regarded as included within thesubject matter of the present description.

The invention claimed is:
 1. A system for cooling an external surface ofa heat conductive body heated by a heat source, the system comprising: atank containing an evaporation liquid having a pressure greater than apressure of an environment near the external surface; a conduit inhydraulic communication with the tank and configured to supply theevaporation liquid to the external surface of the heat conductive body;a controllable supply valve arranged within the conduit, thecontrollable supply valve configured for regulating a flow rate ofegress of the evaporation liquid from the tank; and a control unitoperatively coupled to the controllable discharge valve, the controlunit configured for controlling operation thereof for supply of theevaporation liquid, the control unit comprising: a temperature sensorarranged within the heat conductive body at a predetermined place andconfigured for producing a temperature sensor signal representative ofthe temperature of the heat conductive body at the predetermined place;and a controller operatively coupled to the temperature sensor and tothe controllable supply valve, the controller being responsive to thetemperature sensor signal and being capable of generating controlsignals for controlling operation of the controllable supply valve byturning the controllable supply on or off, thereby to provide a pulsedsupply of the evaporation liquid to the external surface of the heatconductive body as long as required for obtaining a desired temperaturedecrease of the heat conductive body at the predetermined place; whereinthe pulsed supply of the evaporation liquid is characterized by aduration Δt of the liquid supply pulses and a valve operating frequency;and wherein the duration Δt of each pulse of said pulsed supply of theevaporation liquid is obtained by Δt=Mc ΔT/LJ, where M is the mass ofthe body, c is the specific heat capacity of the body, ΔT is the desiredtemperature decrease; L is the latent heat of evaporation of theevaporation liquid, and J is the flow rate of the evaporation liquidthrough the conduit.
 2. The system of claim 1, wherein the control unitincludes a flow meter and a flow rate valve arranged in the conduit; theflow meter configured for measuring the flow rate of the evaporationliquid in the conduit, and the flow rate valve configured to regulatethe flow rate.
 3. The system of claim 1, wherein an operating frequencyof the controllable supply valve is obtained by f=W/LJΔt, where W is theheat power of the heat source.
 4. A system, for cooling an externalsurface of a heat conductive body heated by a heat source, the systemcomprising: a tank containing an evaporation liquid having a pressuregreater than a pressure of an environment near the external surface; aconduit being in hydraulic communication with the tank and configured tosupply the evaporation liquid to the external surface of the body; acontrollable supply valve arranged within the conduit, and configuredfor regulating a flow rate of egress of the evaporation liquid from thetank; and a control unit operatively coupled to said controllabledischarge valve, and configured for controlling operation thereof forsupply of the evaporation liquid, the control unit comprising: atemperature sensor arranged within the body at a predetermined place andconfigured for producing a temperature sensor signal representative ofthe temperature of the body at the predetermined place; and a controlleroperatively coupled to said temperature sensor and to said controllablesupply valve, said controller being responsive to said temperaturesensor signal and being capable of generating control signals forcontrolling operation of said controllable supply valve by turning it“on” or “off”, thereby to provide a pulsed supply of the evaporationliquid to the external surface of the body as long as required forobtaining a desired temperature decrease of the body at thepredetermined place; wherein an inner volume of the conduit between thecontrollable supply valve and an opening through which the evaporationliquid is supplied to the external surface of the heat conductive bodyhas a predetermined value obtained by ΔV≈McΔT/Lρ, where M is the mass ofthe heat conductive body, c is the specific heat capacity of the heatconductive body, ΔT is the desired temperature decrease, L is the latentheat of evaporation of the evaporation liquid and ρ is the density ofthe evaporation liquid.
 5. The system of claim 1, further comprising agrid arranged over the external surface of the heat conductive body at apredetermined distance, and configured for catching the evaporationliquid provided by the conduit, and holding the evaporation liquid in agap between the grid and the external surface.
 6. A method for coolingan external surface of a heat conductive body, the method comprising:providing a system comprising: a tank containing an evaporation liquidhaving a pressure greater than a pressure of an environment near theexternal surface; a conduit being in hydraulic communication with thetank and configured to supply the evaporation liquid to the externalsurface of the body; a controllable supply valve arranged within theconduit, and configured for regulating a flow rate of egress of theevaporation liquid from the tank; and a control unit operatively coupledto said controllable discharge valve, and configured for controllingoperation thereof for supply of the evaporation liquid, the control unitcomprising: a temperature sensor arranged within the body at apredetermined place and configured for producing a temperature sensorsignal representative of the temperature of the body at thepredetermined place; and a controller operatively coupled to saidtemperature sensor and to said controllable supply valve, saidcontroller being responsive to said temperature sensor signal and beingcapable of generating control signals for controlling operation of saidcontrollable supply valve by turning it “on” or “off”, thereby toprovide a pulsed supply of the evaporation liquid to the externalsurface of the body as long as required for obtaining a desiredtemperature decrease of the body at the predetermined place; controllingoperation of the controllable supply valve by turning the controllablesupply valve on or off, thereby providing a pulsed supply of theevaporation liquid to the external surface of the heat conductive bodyas long as required for obtaining a desired temperature decrease of theheat conductive body at the predetermined place; wherein said pulsedsupply of the evaporation liquid is characterized by a duration Δt ofthe liquid supply pulses and a valve operating frequency; and whereineach pulse of said pulsed supply of the evaporation liquid is obtainedby Δt=Mc ΔT/LJ, where M is the mass of the body, c is the specific heatcapacity of the body, ΔT is the desired temperature decrease; L is thelatent heat of evaporation of the evaporation liquid and J is the flowrate of the evaporation liquid through the conduit.
 7. The method ofclaim 6, wherein an operating frequency of the controllable supply valveis obtained by f=W/LJΔt, where W is a heat power of the heat source.